This disclosure relates generally to patient monitoring. More particularly, the present invention relates to control of alarm generation in physiological monitoring apparatuses, termed patient monitors below.
Patient monitors are electronic devices designed to display physiological information about a subject. Electrocardiogram (ECG), electroencephalogram (EEG), plethysmographic signals, and signals related to blood pressure, temperature, and respiration represent typical physiological information contained in full-size patient monitors. Patient monitors are typically also furnished with alarming functionality to alert the nursing staff when a vital sign or physiological parameter of a patient exceeds or drops below a preset limit. Alarms are normally both audible and visual effects aiming to alert the staff to a life-threatening condition or to another event considered vital.
In addition to individual sensor/parameter alarms, patient monitors may be configured to raise combinatory alarms. That is, several physiological parameters may be used to determine a combined index and to give an alarm when the combined index fulfills a specific criterion. The combinatory alarms may range from simple combinations like “low heart rate and low arterial pressure” to complex rule-based scenarios used in various clinical support systems, for example. Below, the term physiological parameter is used to refer to the physiological variable to be monitored. As discussed above, the variable may be an individual parameter, such as heart rate or blood pressure, or a combinatory variable/index derived from multiple individual parameters. An individual physiological parameter may also represent a waveform signal value determined over a predefined period of time.
In most monitors, the alarm limits of a physiological parameter may be defined by the user, since the limits typically depend on patient etiology, age, gender, medication, and various other subjective factors. Each physiological parameter may also be assigned more than one alarm limit/criterion. That is, for a specific physiological parameter a patient monitor may raise alarms of different levels.
The monitor may also be provided with an alarm escalation mechanism to escalate unacknowledged and/or persistent alarms. Typically, an alarm is raised when the physiological parameter reaches a predefined first alarm limit, and the level of priority/severity of the alarm is increased if the alarm persists and/or remains unacknowledged for a predetermined period of time. The escalation rules stored in the monitor define how an alarm is escalated. Each alarm level may involve different alarming functionality.
In a clinical environment, one problem is alarming for weak signals. Many of the physiological signals measured from the patient are weak in nature and thus also extremely vulnerable to interference. Furthermore, the level of a physiological signal may drop significantly due to an external reason. For example, the level of an ECG signal may drop when the patient turns in bed or the level of a plethysmographic signal may drop when the patient is subject to a clinical operation that causes pain. Monitoring such weak signals in a noisy environment makes alarm generation a demanding task, as the monitor should be both sensitive and specific in raising alarms. That is, the monitor should be able to recognize all true alarm events, without raising false or clinically irrelevant alarms. During low signal level, the patient monitor should be able to detect real alarm events in which the signal level is low, such as asystole, but if the patient monitor is made sensitive enough for such events, noise and interference tend to increase the number of false alarms, or worse, cause the monitor to miss a real asystole alarm due to noise triggering the heart heat counter. Nuisance alarms are thus increased when the level of the signal is lower than normal, since the patient monitor may alert of the lowered signal level (regarded as a possible alarm) and since the quality/confidence of the parameter calculation is lowered during periods of lowered signal level. Due to the lowered quality/confidence, the number of events of the physiological parameter only slightly crossing the lowest priority threshold, and thus also the number of clinically irrelevant alarms, is increased during periods of lowered parameter confidence.
Due to the above difficulties, in a real clinical environment a large fraction of the alarms, even most alarms, may be false or at least clinically irrelevant. Such a large number of false or irrelevant alarms causes an enormous burden on the nursing staff and may also lead to impairment of the responses to true alarms. Therefore, reducing the amount of false or clinically irrelevant alarms is central to patient monitoring.